Issue 38

I. N. Shardakov et alii, Frattura ed Integrità Strutturale, 38 (2016) 331-338; DOI: 10.3221/IGF-ESIS.38.43 334         0 0 , , , / , 100% j j C x y TN x y T x y T x y        In order to make a decision on the existence of bebonding at the point with the coordinates   , x y , we calculate a threshold value for the temperature contrast * C . For making it estimate, we determine, at each loading step, the average value j C and the standard deviation j  in those areas of the thermograms, where debonding is known to be absent. The threshold value is calculated by the formula * 3 j j С С    . The areas of the CRFP layer surface, where the temperature contrast exceeds the threshold value, are identified as the areas with debonding and the remaining ones as the areas free of defect. In the binary defect map shown in Fig.3d, the defect-free areas are shown in white color and the areas of debonding in black color. (a) (b) (c) (d) Figure 3 : Thermal infrared images obtained using the developed algorithm: (а) initial thermogram; (b) normalized thermogram; (c) temperature contrast map; (d) binary card of defects. R ESULTS AND DISCUSSIONS he summary data of the static test results for the beams of the series A and B are shown in Tab. 1, the series C – in Tab. 2, where M crc is the bending moment corresponding to the onset of cracking, a crc, is the maximum crack opening width, f ult is the elastic deflection, M ult is the maximum bending moment, f ult is the maximum deflection, ε f,ult is the strain of the carbon-fiber sheet at rupture. Before tests the class of concrete was specified for each beam sample. For the marking of samples were used the following notation: B1 or B2 – groups of concrete, "a", "b", "c" – series, i – sample number. During the tests we observed two forms of delamination. For the beams, whose surface had been cleaned with a wire brush before sticking CFRP, the delamination occurred according to the adhesive scenario. For the beams, refined with an abrasive tool to a depth of 2-3 mm, the delamination occurred according to the cohesive scenario. For the non- strengthened beams (series A) the destruction state was determined by the rupture of metal reinforcement rods and crushing of the concrete in the compressed zone, for the strengthened ones (series B and C) – by the rupture of CFRP layer, in a number of cases accompanied by the rupture of metal reinforcement. Fig. 4 shows the dependence of the maximum beam deflection on the bending moment obtained for three series of beams. The comparison of the graphs obtained in the series A and B clearly demonstrates an increase in the bearing capacity of the beams strengthened before loading. The maximum bending moment, which such beams can stand, has happened to be by 37–39% higher than the reference samples. The graphs reflect the appearance of the first cracks in the concrete: it corresponds to a sharp change in the slope angle of the curves. T